The present invention relates to a signal selecting device used in transmission, reception or transmission/reception of information. In the field of radio communication using radio waves, necessary signals and unnecessary signal are separated by extracting signals at a particular frequency from a large number of signals. Filters that perform this function comprise a resonator and an impedance transforming circuit and are incorporated in many radio devices. Such filters cannot change design parameters, such as the center frequency and the bandwidth. Therefore, a radio communication device using a plurality of combinations of center frequencies and bandwidths has to have a number of filters equal to the number of combinations of center frequencies and bandwidths and select a filter for use by means of a switch or the like. For example, a non-patent literature 1 (DoCoMo Technical Journal Vol. 14, No. 2, pp. 31-37) discloses a related art in which a filter for use is selected from among a plurality of filters by means of a switch.
Related arts, such as that disclosed in the non-patent literature 1, have a problem that, as the number of combinations of center frequencies and bandwidths increases, the circuit area and the number of components also increase. An object of the present invention is to provide a filter capable of appropriately changing a center frequency and a bandwidth by controlling characteristics of a resonator and an impedance transforming circuit and to reduce the number of filters used even when a plurality of combinations of center frequencies and bandwidths is used.